Android基础面试题(二)

4 Fragment

4.1 Fragment的使用

4.2 FragmentTransaction

4.3 Fragment与Activitiy之间的通信

4.3 Fragment的回退栈

Fragment懒加载

Fragment与View的状态保存与恢复

Fragment与大型数据缓存

5 线程池

5.1 线程池的优点

.重用线程池中的线程,避免因为线程的创建和销毁所带来的性能开销
.能有效控制线程池的最大并发数,避免大量的线程之间因互相抢占系统资源而导致的阻塞现象
.能够对线程进行简单的管理,并提供定时执行以及指定时间间隔循环执行等功能

5.2 Api实现的四种不同特性的线程池

Android中利用ThreadPoolExecutor来实现不同特性的线程池.分别为:FixThreadPool,CachedThreadPool,SingleThreadExecutor,ScheduleThreadPool

5.2.1 SingleThreadExecutor

• 特点:只有一个核心线程(保证所有任务按照指定顺序在一个线程中执行,不需要处理线程同步的问题)
• 应用场景:不适合并发
• 具体使用:Executors.newSingleThreadExecutor()
• 具体的定义:
  /**
  • Creates an Executor that uses a single worker thread operating
  • off an unbounded queue. (Note however that if this single
  • thread terminates due to a failure during execution prior to
  • shutdown, a new one will take its place if needed to execute
  • subsequent tasks.) Tasks are guaranteed to execute
  • sequentially, and no more than one task will be active at any
  • given time. Unlike the otherwise equivalent
  • {@code newFixedThreadPool(1)} the returned executor is
  • guaranteed not to be reconfigurable to use additional threads.
  • @return the newly created single-threaded Executor
    */
    public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
    (new ThreadPoolExecutor(1, 1,
    0L, TimeUnit.MILLISECONDS,
    new LinkedBlockingQueue()));
    }

5.2.2 FixThreadPool(定长线程池)

• 特点:只有核心线程&不会被回收,线程数量固定,任务队列无大小限制(超出的线程任务会在队列中等待)
• 应用场景:控制线程最大并发数
• 具体使用:Executors.newFixedThreadPool()来创建
• 具体的定义:

      /**
     * Creates a thread pool that reuses a fixed number of threads
     * operating off a shared unbounded queue.  At any point, at most
     * {@code nThreads} threads will be active processing tasks.
     * If additional tasks are submitted when all threads are active,
     * they will wait in the queue until a thread is available.
     * If any thread terminates due to a failure during execution
     * prior to shutdown, a new one will take its place if needed to
     * execute subsequent tasks.  The threads in the pool will exist
     * until it is explicitly {@link ExecutorService#shutdown shutdown}.
     *
     * @param nThreads the number of threads in the pool
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

5.2.3 ScheduleThreadPool(定时线程池)

• 特点:核心线程数量固定,非核心线程数量无限制(闲置时马上回收)
• 应用场景:执行定时,周期性任务
• 具体实现:Executors.newScheduledThreadPool()创建
• 具体的定义:
  /**
  • Creates a thread pool that can schedule commands to run after a
  • given delay, or to execute periodically.
  • @param corePoolSize the number of threads to keep in the pool,
  • even if they are idle
  • @return a newly created scheduled thread pool
  • @throws IllegalArgumentException if {@code corePoolSize < 0}
    /
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
    }
    /*
  • Creates a new {@code ScheduledThreadPoolExecutor} with the
  • given core pool size.
  • @param corePoolSize the number of threads to keep in the pool, even

  •    if they are idle, unless {@code allowCoreThreadTimeOut} is set
    

  • @throws IllegalArgumentException if {@code corePoolSize < 0}
    */
    public ScheduledThreadPoolExecutor(int corePoolSize) {
    super(corePoolSize, Integer.MAX_VALUE,
    DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
    new DelayedWorkQueue());
    }

5.2.4 CachedThreadPool(可缓存线程池)

• 特点:只有非核心线程,线程数据不固定(可无限大),灵活回收空闲线程(具备超时机制,全部回收时不占系统资源).任何线程任务到来都会立刻执行,不需要等待
• 应用场景:执行大量,耗时少的线程任务
• 具体实现:Executors.newCachedThreadPool()
• 具体的定义
  /**
  • Creates a thread pool that creates new threads as needed, but
  • will reuse previously constructed threads when they are
  • available. These pools will typically improve the performance
  • of programs that execute many short-lived asynchronous tasks.
  • Calls to {@code execute} will reuse previously constructed
  • threads if available. If no existing thread is available, a new
  • thread will be created and added to the pool. Threads that have
  • not been used for sixty seconds are terminated and removed from
  • the cache. Thus, a pool that remains idle for long enough will
  • not consume any resources. Note that pools with similar
  • properties but different details (for example, timeout parameters)
  • may be created using {@link ThreadPoolExecutor} constructors.
  • @return the newly created thread pool
    */
    public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
    60L, TimeUnit.SECONDS,
    new SynchronousQueue());
    }

5.3 自定义线程池

• Android线程池的概念来自于Java的Executor接口,此接口的真正的实现类为ThreadPoolExecutor
• ThreadPoolExecutor的构造方法提供了一些参数来配置线程池.

5.3.1 具体的构造方法

public ThradPoolExecutor(int corePoolSize,int maximumPoolSize,
    long keepAliveTime,TimeUnit unit,
    BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)

5.3.2 参数的理解

• corePoolSize:核心线程数,默认情况下,核心线程即使闲置也会一直存活.但是可以通过设置allCoreThreadTimeOut为true,来允许其等待超时,并用keepAliveTime来设置超时时间
• maximumPoolSize:线程池所能容纳的最大线程数,当活动线程数达到这个数值后,后续的新任务将会被阻塞.
• keepAliveTime:非核心线程的超时时间.当然设置了allCoreThreadTimeOut为true时,也会作用于核心线程.
• unit:用于指定keepAliveTime参数的时间单位,是一个枚举类型.常用的有TimeUnit.MILLISECONDS和TimeUnit.SECONDS
• workQueue:线程池中的任务队列,通过线程池的execute方法提交的Runnable对象会存储在这个参数中
  1. ArrayBlockingQueue:基于数组结构的有界阻塞队列,此队列按FIFO(先进先出)原则对元素进行排序.
  2. LinkedBlockingQueue:基于链表结构的阻塞队列,此列表按FIFO(先进先出)排序元素,吞吐量要高于ArrayBlockingQueue.静态工厂方法Executors.newFixedThreadPool()使用了这个队列
  3. SynchronousQueue:一个不存储元素的阻塞队列.每个插入操作必须等到另一个线程调用移除操作,否则插入操作一直处于阻塞状态.吞吐量要高于LinkedBlockingQueue,静态工厂方法Executors.newCachedThreadPool使用了这个队列
  4. PriorityBlockingQueue:一个具有优先级的无线阻塞队列.
• threadFactory:线程工厂,为线程池提供创建新的线程的功能.threadFactory是一个接口,只有一个方法:public abstract Thread newThread(Runnable r)
• RejectedExecutionHandler: 拒绝策略.线程池已经关闭或者是最大线程数和任务队列已经饱和,无法再接收新的任务时,在使用execute()来提交新的任务时将会拒绝.而默认的策略是抛一个RejectedExecutionException异常.

5.3.3 执行任务时的策略

以下用currentSize表示线程池中当前线程数量

1. currentSize<corePoolSize,将会直接启动一个核心线程来执行任务
2. currentSize>=corePoolSize,任务会被插入到任务队列中排队等待执行
3. 任务队列已满,currentSize<maximumPoolSize,会立刻启动一个非核心线程来执行任务
4. 任务队列已满,currentSize>maximumPoolSize,会拒绝执行任务,ThreadPoolExecutor会调用RejectedExecutionHandler的rejectedExecution方法来通知调用者

5.3.4 使用

1. 使用构造函数创建对象
2. 使用execute(Runnable runnable)向线程池提交任务
3. 使用shutdown()方法来中断(interrupt())所有没有正在执行任务的线程
   使用shutdownNow()来停止(stop)所有正在执行或暂停任务的线程.
   建议:一般调用shutdown()来关闭线程池;若任务不一定要执行完,则调用shutdownNow()

参考到地址
基础讲的比较好的一篇:https://www.jianshu.com/p/0e4a5e70bf0e
结合代码进行讲解:https://www.jianshu.com/p/7b2da1d94b42
讲解的特别基础好理解:https://blog.youkuaiyun.com/l540675759/article/details/62230562
参考书籍:Android开发艺术探索

4 AsyncTask的缺陷和问题，说说他的原理

4.1 基础知识

4.2 缺陷

鸡汤一则:积累是一个漫长的过程,需要的是不断的刷新与整理.最终才能由量变到质变!